Mutable defaults

[J. Pic]

Silly me, we don't even need copy but just execution

@lazy(x=lambda: [], y=lamba x: len(x))
def foo(x=None, y=None):

So only one operator is needed, the walrus is still fine:

def foo(x:=[], y:=len(x)):

Not copy, just evaluate.

Le jeu. 11 févr. 2021 à 03:54, J. Pic <jpic at yourlabs.org> a écrit :

> Adding decorators with some inspect sauce could certainly work with the
> syntax we already have:
>
> @default(x=lambda: copy([]), y=lambda x: len(x))
> def foo(x=None, y=None):
>
> I think this PoC is doable. But the lambda copy is a boring so, two
> decorators:
>
> @default.copy(x=[])
> @default.call(y=lamba x: len(x))
> def foo(x=None, y=None):
>
> Or with a new syntax:
>
> def foo(x:=[], y`=len(x))
>
> Here walrus is used to denote that an assignment of the right side is
> going to take place at call, and the backtick to denote that execution of
> the right side is going to take place.
>
> Le jeu. 11 févr. 2021 à 03:06, Chris Angelico <rosuav at gmail.com> a écrit :
>
>> On Thu, Feb 11, 2021 at 12:56 PM J. Pic <jpic at yourlabs.org> wrote:
>> >
>> > I just meant removing the whole "default value mutating" story, not
>> removing mutable variables. Really, I was wondering if there was a use case
>> where this actually turns to an advantage, in which case it would be a
>> designed feature rather than an undesirable side effect, which it seems to
>> be.
>> >
>>
>> Right, I know you weren't asking about removing mutables altogether.
>> But how would you remove "default value mutating"? Do you disallow any
>> mutable values from being argument defaults? Because that's a huge
>> category of values that are no longer available.
>>
>> Mutable defaults most certainly ARE used deliberately, although often
>> it's more as a sort of "static variable" rather than actually a
>> parameter. But it's common enough as an idiom that it can't be changed
>> without breaking a lot of code.
>>
>> > Now I'm wondering if you have considered copy on write, to save the
>> overhead from copying every mutable default at every call when it is not
>> necessary.
>> >
>>
>> That's its own level of mess.
>>
>> The only way to move forward with this is something
>> backward-compatible, which probably means an explicit request for late
>> binding (or copying, which comes to much the same thing). But it needs
>> an elegant syntax. The proposal lives or dies on that syntax, and so
>> far, it's always died.
>>
>> ChrisA
>> --
>> https://mail.python.org/mailman/listinfo/python-list
>>
>